Cellulose digester with washing apparatus



' Oct. 28, 1969 o. A. LAAKS O CELLULOSE DIGESTER WITH WASHING APPARATUS Filed Feb. 7, 1966 04 1/5? 1 away United States Patent 3,475,271 CELLULOSE DIGESTER WITH WASHING APPARATUS Oliver A. Laakso, Glens Falls, N.Y., assignor to Aktiebolaget Kamyr, Karlstad, Sweden Filed Feb. 7, 1966, Ser. No. 525,653 Int. Cl. D21c 7/00 US. Cl. 162-242 13 Claims ABSTRACT OF THE DISCLOSURE An apparatus is provided for the fiber-liberating digestion of continuously fed, comminuted cellulosic fiber material by digesting liquid and the subsequent washing of the liberated fiber material by at least partially displacing the spent digesting liquor with washing liquid. The washing of the digested fiber is effected by directing radially inward in the digester a substantially horizontal flow of washing liquid whereby the digesting liquor is displaced into a central strainer body located at one end and inside the digester.

The present invention relates to improved apparatus for the fiber-liberating digestion of continuously fed, comminuted cellulosic fiber material by digesting liquid and the subsequent washing of the liberated fiber material by at least partially displacing spent digesting liquor with washing liquid.

Cellulose digesters of the prior art have performed a preliminary wash of the digested pulp before the pulp is discharged from the digester. However, when the fiber material introduced into the digester is sawdust or some other finely comminuted fiber material, the digestion produces a very compact pulp with a consequentially high resistance to the flow of wash Water therethrough.

An object of this invention is to provide means in a cellulose digester for uniform distribution of a wash liquid throughout a zone of the digester having a considerable axial extent. Another object of this invention is to provide apparatus in a cellulose digester to enable digesting liquor which has been displaced by wash liquid to be strained from the pulp using a strainer which reduces the clogging of the strainer apertures by the fiber material. A further object of this invention is to provide wash means in a cellulose digester to allow washing of the digested fiber material by a washing liquid which flows from the shell of the digester radially, and substantially horizontally inwards over a substantial portion of the digester near the digester outlet end while displacing spent digesting liquor which is discharged through a hollow strainer body centrally located in the lower portion of the digester to a discharge conduit. Still another object of this invention is to provide apparatus which will divide the spent digesting liquor of a cellulose digester into different fractions. A still further object of this invention is to provide apparatus for the digestion and Washing in a digestion vessel of finely comminuted cellulosic fiber material. Still further objectives and the entire scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes or modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.

The above objects of this invention are obtained by providing a cylindrical strainer girdle with connected wash liquid conduits in the shell of a cellulose digester close to the outlet end thereof, and a central hollow circular strainer body located coaxially to and about the same level 3,475,271 Patented Oct. 28, 1969 as the strainer girdle and having substantially the same height as the strainer girdle and a smaller diameter than the digester shell. The central strainer body is attached to a rotary shaft and connected to a discharge conduit for spent liquid. Over the entire space between the concentric strainer faces, a substantially horizontal and radially inwards directed flow of wash liquid and digesting liquor displaced thereby can be maintained.

The invention will be more closely described hereinbelow with reference to the accompanying drawing, in which FIGURE 1 diagrammatically illustrates the cellulose digester of this invention, and FIGURE 2 shows a modified embodiment of the lower end of the digester.

Referring to FIGURE 1, a fiber material, for example, sawdust, is supplied through a hopper 13 and rotary valve 15, to an upright, substantially cylindrical digester 11, adapted for the continuous downward passage of cellulosic fiber material while subjecting the fibrous material to a fiber-liberating digestion by means of a suitable digesting liquor at elevated pressures and temperatures. The fiber material is generally supplied to the digester mixed with the digesting liquor, for example, sulphate liquor. The fiber material mixed with the digesting liquor passes through the rotary valve 15 into a steaming vessel 17 where the material is steam-heated while agitated by the conveyor screw 19. The fiber material then passes through a rotary valve 21 and inlet neck 23 into the upper end of the digester 11. The valve 21 is flushed by steam from steam vessel 25. The above-described feeding means are conventional and are described, for example, in US. Patent No. 2,914,225. If needed, an additional quantity of steam may be supplied directly to the digester to raise the temperature of the incoming fiber material to the temperature required for the digestion step, and to produce the desired pressure-e.g., temperatures in the order of ISO-200 C. and pressures in the order of 8 atmospheres gauge pressure.

At its lower end, the digester 11 has a central outlet 27 provided with a scraping device 31 attached to a vertical shaft 33 located centrally in the digester bottom and driven by a gear 35 by means of a motor (not shown). The scraping device 31 can be controlled in such a manner that the digester is maintained filled with fiber mate rial to the level 29, above which mainly steam is present. Such fiber material discharge means are conventional and may correspond, for example, to those shown in U. S. Patent No. 2,938,824.

The washing zone of the improved digester of this invention is particularly designed for overcoming the difficulty encountered by the prior art in the compaction of the pulp with the consequential high resistance to wash water flow. The apparatus of this invention allows washing in the digester wherein the wash liquid has a quite short flow path over a quite large cross-sectional area, whereby the washing step may be performed at a comparatively low flow rate of wash liquid.

According to the invention, there is provided at the outlet end of the digester a central strainer body 37, generally extending along the axis of the digester and having a cylindrical perforated or slitted strainer shell 39. The axial length of the central strainer body 37 is at least of the same order as the diameter of the digester shell and is preferably of a diameter which is from at least equal to 2 or 3 times greater than that of the digester shell. The diameter of the central strainer body 37 is preferably /s to /a that of the diameter of the digester. With the preferred dimensions, the total surface of the central strainer body will be at least equal to the cross-sectional area of the digester and preferably will be considerably greater in area. By total surface area of the central strainer body is meant the total surface 3 of the cylindrical shell 39 and not merely the total surface of the apertures therein.

The wash liquid required for the preliminary washing of the pulp and the displacement of spent digestion liquor may consist of fresh water or a filtrate fraction from a filter upon which the pulp is washed after discharge from the digester. This wash liquid is supplied at and is'distributed around the circumference of the lower-most cylindrical portion of the digester, which section is preferably of somewhat greater outer diameter than the uppermost portion of the digester. The wash liquid is supplied through wash liquid conduit fittings 41 and 43 through the shell of the lower portion of the digester into the space between the lower portion of the shell of the digester and a cylindrical slitted or perforated strainer girdle 45.

This strainer girdle 45 is preferably of the same inner diameter as the upwardly adjoining portion of the digester shell so that it does not form an obstacle to the descent of the column of fiber material. The strainer girdle 45 is located essentially at the same level as and has essentially the same axial extension, or length, as the strainer shell 39 of the central strainer body 37. Thus, the strainer girdle 45 supplies wash liquid to the digester 11 in a manner such that the wash liquid is introduced into the digester as a flow composed of horizontal, radially directed jets distributed over essentially the entire face of the strainer girdle 45. Each portion of the wash liquid fiow finds its way along the shortest possible path to the central strainer body 37, driving spent liquor ahead of itself out of the pulp and into the central strainer body 37. The interior of the central strainer body 37 communicates through a bore in the vertical shaft 33 with a discharge conduit 47, through which the liquid from the central strainer body is carried away from the digester, for example, to a recovery plant. The pressures maintained outside the strainer girdle 45 and in the discharge conduit 47 are chosen, relatively to the pressure of the digester, such that at least the greater part of the spent liquor has been replaced by wash liquid when the pulp, after having passed the zone between the strainer girdle 45 and the central strainer body 37, is about to be discharged from the digester. If desired, an excessive wash can be performed, so that wash liquid is allowed to penetrate into the central strainer body and to mix there with the spent liquor.

Ordinarily, a more or less pronounced conical border surface 49 will develop between the spent liquor and the wash liquid. On account of the axial downward motion of the pulp the resulting liquid flow may deviate somewhat from the horizontal direction, which, if desired, can be compensated by displacing one of the strainers 39, 45 relatively to the other in the axial direction, so that they are located only partly opposite each other at the same level. In order to avoid displacement of the digesting liquor relatively to the pulp in the part of the digester located above the washing zone, the quantity of wash liquid supplied and the quantity of liquid discharged through the conduit 47 should be substantially equal. If it is desired to dilute the pulp in order to facilitate its discharge, the required quantity of diluting water preferably is introduced below the scraping device 31.

The central strainer body 37 is attached to the vertical shaft 33 of the scraping device and thus rotates at the same speed as the scraping device 31. As the pulp does not take part in the rotation to any appreciable extent until it reaches the scraping device 31, there is relative movement between the outer face of the central strainer body 37 and the pulp. This movement prevents fiber particles from sticking to the face of the strainer shell 39 and also prevents the formation of a compact pulp layer adjacent to the face of the strainer shell 39, which layer if formed could choke the flow of liquid through the strainer shell 39.

Although the strainer body 37 may be carried by the free end of the vertical shaft 33 of the scraping device, it is preferably supported, as shown in FIGURE 1, by an upper journal extending through the top of the digester 11 and supported by a bearing 53. A filling-out body 55 which is of the same diameter as the central strainer body 37 may be inserted between the strainer body 37 and the upper journal 51 to provide a uniform cross-sectional area throughout the digester for How of the fiber material pulp so that the pulp is unobstructed by the central strainer body 37. Optionally, the filling-out body 55 may be stationary, in which case the central strainer body 37 is journaled into the lower end of the filling-out body 55.

FIGURE 2 illustrates an embodiment of the cellulose digester of this invention wherein the space between the lower, wider portion of the digester shell and the strainer girdle 45 is divided in the axial direction by means of an annular horizontal partition 57. The central strainer body 37 is also divided up by an interior horizontal partition 59. The two cavities inside the central strainer body 37 divided by the interior horizontal partition 59 have separate outlets through the shaft 33. Conduit 47 connected to the upper cavity functions as in FIGURE 1 to provide passage for the spent liquor that has been displaced by wash liquid supplied through wash liquid conduit fitting 41. The spent liquor in the upper cavity is not mixed to any significant extent with the wash liquid and, therefore, maintains substantially its full concentration and temperature, so that its contents, both heat and chemical, can be economically recovered. The liquid in the lower cavity of the central strainer body 37 generally consists of wash liquid mixed with a minor amount of spent digesting liquor and has a temperature which is considerably below the digestion temperature. This liquid is difiicult to process in a recovery plant but may be used as wash liquid by supplying it by means of conduit 60 and recycle means 61 to the wash liquid conduit fitting 41, whereby a two-stage counter-current wash in the digester is obtained. Additional partitions may be provided to divide the space between the digester shell and the strainer girdle and the space inside the central strainer body into three or more cavities.

Instead of being cylindrical, the strainer shell 39 of the central strainer body 37 may be somewhat conical, as shown in FIGURE 2. The rotation of the strainer body 37 and its closeness to the scraping device 31 overcomes the tendency of the pulp to form plugs in the convergmg space between the strainer faces 45 and 39. In addition, the cylindrical filling-out body 55 may be replaced by a conically pointed endpiece forming an extension of the central strainer bod and extending somewhat above the strainer faces 45 and 39. While the digestion vessel of the present invention has been described with the feed means at the upper end of the vessel and fiber discharge means at the lower end thereof, it should be understood that the present invention is not limited to any particular digester position. The described digester vessel may be turned upside down, if desired.

I claim:

1. In an apparatus for the fiber-liberating digestion of continuously fed comminuted cellulosic fiber material by digesting liquid, comprising longitudinal, essentially cylindrical, upright cellulose digestion vessel, feed means at one end of the vessel and fiber discharge means at the other end of the vessel, the improvement comprising a cylindrical strainer girdle in the shell of the digestion vessel near the fiber discharge means end of the vessel, a rotary vertical shaft located centrally attached at the said discharge means of the digestion vessel between the discharge end of the central strainer body and the said discharge means of the digestion vessel, a conduit for the supply of wash water connected to the strainer girdle, a central strainer body of essentially circular cross-section coaxial to and at about the same level as the strainer girdle wherein the central strainer body has essentially the same extension in height in the vertical direction as the strainer girdle, has a diameter smaller than the shell of the digestion vessel, is attached to the vertical shaft and, a discharge conduit for spent liquor connected to the interior of said central strainer body by a bore in said vertical shaft, whereby an essentially horizontal, radially inward flow of wash water and displaced digesting liquor can be maintained over essentially the entire space between the concentric strainer girdle and strainer body.

2. The apparatus of claim 1 wherein the central strainer body has a diameter of /5 to /3 that of the shell of the digestion vessel and the strainer girdle has a diameter about that of the shell of the digestion vessel.

3. The apparatus of claim 1 wherein the central strainer body and the strainer girdle have axial lengths of at least the diameter of the shell of the digestion vessel and not exceeding substantially three times the diameter of said shell.

4. The apparatus of claim 1 wherein a rotating scraping device is connected to said vertical shaft in the fiber discharge means end of the digestion vessel to forward fiber material towards the fiber discharge means end.

5. The apparatus of claim 1 wherein the total surface area of the central strainer body is at least equal to the cross-sectional area of the digestion vessel.

6. The apparatus of claim 1 wherein the central strainer body contains at least one annular, essentially horizontal partition means to define at least two portions, each portion provided with separate discharge outlets.

7. The apparatus of claim 6 wherein the strainer girdle contains at least one annular, essentially horizontal partition means to define at least two portions, and means for connecting the strainer girdle portions furthest from the fiber discharge means end are connected to the discharge outlets of the portions of the central strainer body nearest the fiber discharge means end, whereby a twostage wash in the digester is obtained.

8. An apparatus for the fiber-liberating digestion of continuously fed comminuted cellulosic fiber material by digesting liquid and the subsequent washing of the fiber material by at least partially displacing spent digesting liquor with wash liquid comprising a longitudinal, essenentially cylindrical, upright digester, feed means located at one end of the digester to feed cellulosic fiber material and digesting liquor to the digester, an essentially cylindrical strainer girdle located in the wall of the digester near the end opposite the feed means end, at least one wash liquid conduit connected to the strainer girdle, means defining a discharge opening in the strainer girdle and of the digester, a discharge conduit for liquid connected to the discharge opening, a rotating vertical shaft located in the strainer girdle end of the digester, scraping means located on the rotating shaft to forward the fiber material towards the discharge outlet, a central strainer body of essentially circular cross-section and of smaller diameter than the digester attached to the rotating shaft and coaxial to and at about the same level in the digester as the strainer girdle, and at least one discharge conduit for spent liquor connected to the strainer body.

9. The apparatus of claim 8 wherein the central strainer body has a diameter of /s to /3 that of the digester and the strainer girdle has a diameter about that of the digester.

10. The apparatus of claim 8 wherein the central strainer body and the strainer girdle have axial lengths of at least equal to the diameter of the shell of the digester vessel and not exceeding substantially three times the diameter of said shell.

11. The apparatus of claim 8 wherein the total surface area of the central strainer body is at least equal to the cross-sectional area of the digester.

12. The apparatus of claim 8 wherein the central strainer body contains at least one annular, essentially horizontal partition means to define at least two portions of the central body, each portion provided with a separate discharge outlet.

13. The apparatus of claim 12 wherein the strainer girdle contains at least one annular, essentially horizontal partition means to define at least two portions of the strainer girdle, and means for connecting the wash liquid conduits of the strainer girdle portions furthest from the strainer girdle end of the digester to the discharge outlets of the central strainer body portions nearest the strainer girdle end, whereby a two-stage counter-current wash in the digester is obtained.

References Cited UNITED STATES PATENTS 439,033 10/1890 Forbes 162246 1,06 1,767 5/1913 Stanley 162-25 1 X FOREIGN PATENTS 150,782 9/ 1920 Great Britain. 458,863 12/ 1936 Great Britain.

S. LEON BASHORE, Primary Examiner U.S. Cl. X.R. 

